Peak shape and pressure

[DeimerlyC]

Hello,

I've never heard of any HPLC training instructor talk about the effects of system pressure on peak shape. So I'm in uncharted terrority here.

On my IC I've noticed that the more pressure I apply to the system, either by ramping up the flow rate or adding restictor columns, my peaks are getting higher. Because I also have a background in GC I'm not surprised. My problem is I think I'm getting too fast for the system and my pressure is not as high as I'd like it to be (it's only around 1000psi).

Does adding pressure actually improve peak shape or am I just wistful thinking?

What can I do to add more pressure? Should I start changing out my connection tubing to smaller IDs?

Thanks in advance

[DR]

Pressure should have very little effect as the VanDeemter curves for LC columns are pretty flat vs. GC columns (think L instead of V).
(now for a wag)
That said, are you altering the supressor countercurrent flow along with your MP flow? Maybe you're optimizing the flow ratio in the supressor? (I won't feel bad if Mark or other IC user corrects me here...).

[tom jupille]

I'll second DR's comments.

To a first approximation, peak height should be independent of pressure. If anything, at higher flow rates (and thus higher pressures), the peaks should broaden a bit and so get somewhat shorter. As you get into "ultra high pressure" (say, around 10,000 psi), things get more complex, but you're way under that range.

[DeimerlyC]

That said, are you altering the supressor countercurrent flow along with your MP flow? Maybe you're optimizing the flow ratio in the supressor? (I won't feel bad if Mark or other IC user corrects me here...).

Thanks for the replies!

OK, you lost me a little bit here at "supressor countercurrent flow." Explain?

Are we talking about optimizing the flow through my pressure regulators? What kind of flow or backpressure should I have to make these little cartridges work more effectively?

Is there a guildeline for how fast your MP should be traveling through the column like in GC, or can I pump however fast I want assuming that eventually I'll pop a line?

[Mark Tracy]

He's talking about ion chromatography with suppressed conductivity detection. The suppressor is a post-column device; it does have a rather narrow optimum pressure range that you need to adjust during installation.

The discussion of optimum flow rate is a bit involved, and you would be better served by reading a good textbook then asking for clarification on the forum. See http://www.sepsci.com/chromforum/viewtopic.php?t=4098 for some suggestions.

[DR]

Thanks, Mark.

P.S. - we have a new IC about to be installed, so I'm sure I (or a coworker) will be peppering the board with questions soon!